of NO_x, HO_x, and HCs in determining the photochemistry of ozone in the upper troposphere. Also, detailed NO_x source inventories, laboratory investigations of key reactions, and emission characterization studies have helped provide input data for assessment models.

The chemistry as outlined in the Brasseur et al. (1997) report is shown in somewhat more detail than that given in the Friedl (1997) report, but where comparisons can be made it appears that there is general agreement on the specific chemistry employed by both groups. It is likely that the chemical mechanisms chosen by the NASA and European groups are very similar, since both U.S. and European groups rely heavily on the two existing major kinetic data reviews, which are very similar in their recommendations (DeMore et al., 1997; Atkinson et al., 1997). In general, SASS's research objectives related to the homogeneous chemistry used in the project's modeling efforts do not appear to be missing any critical elements. However, there are some issues the panel feels may deserve more attention, as discussed in the following sections.

The nature and extent of the chemistry included in the assessment models should ideally be based on our knowledge of the aircraft emissions under actual flight conditions. In the absence of such knowledge, choices are made on the basis of measurements made during bench tests of aircraft engines. Observations of exhaust plume components seem to have been restricted to those components that are believed, a priori, to be important (e.g., NO_x, CO, SO_x, soot, aerosols). Friedl (1997) suggests strongly that this information is still very incomplete. As discussed below, there are some issues related to the emissions of hydrocarbons and nitrogen-containing compounds that may deserve particular attention; discussion of sulfur-compound emissions is reserved for the 'aerosols' section of this report.

Although current jet-engine bench tests indicate that aircraft engine combustion is very efficient under many operating conditions, combustion in any engine is never "complete." Some unburned fuel is found in jet-aircraft exhaust, particularly when a fuel-rich mixture is used in the engine. Friedl (1997) notes on p. 36, "Aircraft exhaust is known to contain a large number of C₂–C₁₇ species, although the relative amounts are not well established." This means that assessments must consider the effects on ozone chemistry of unburned hydrocarbons and hydrocarbon oxidation products in the exhaust plume.

Our knowledge of the hydrocarbons present in current jet fuels is reasonably good, but some important minor components remain ill-defined. JP-8 fuel, which is used in many units of the U.S. Air Force, complies with a set of specifications that are essentially identical with those of civilian aviation fuel (JA-1), except for fuel additives required by the JP-8 specification (CRC, 1984). Mayfield (1996) reports that on average the JP-8 jet fuel mixture consists of about 80.4% alkanes

Front Matter (R1-R12)

Executive Summary (1-3)

1 Context for this Review (4-8)

2 Discussion of the Science Issues (9-26)

3 Modeling Considerations (27-30)

4 Priority Concerns and Recommendations (31-34)

References (35-39)

Acronyms (40-41)